1. Field of the invention
This invention relates in general to packoff assemblies located between a casing hanger and a wellhead, and in particular to a packoff assembly for a subsea well employing a metal seal.
2. Description of the Prior Art
In drilling subsea wells, concentric casings are hung. These protect the higher strata from the pressure and fluids which may be encountered at greater depths. Casing hangers are concentrically located to support the various casing strings.
It is necessary to contain the pressure within the inner casings so that the annular space outside of the inner casing does not receive the high pressure. For this purpose, it is known to pack off or seal annular space between the hanger bodies.
A particular seal for packing off the annular opening is shown in U.S. Pat. No. 3,797,864 Hynes et al, issued Mar. 19, 1974. This seal comprises an elastomeric ring interposed between two metallic rings. Each ring has a pair of lips extending toward the elastomer so that on axial compression the lips are forced outwardly into contact with the walls. A disadvantages of this packoff is that to seal high pressure, a high compressive load is required. This high compressive load is normally generated by the rotation of a packoff nut. High torque is required, which is further increased by the friction in the thread mechanism. Another problem is faced when the packoff is retrieved. The pulling from the upper ring often results in a rupturing or tearing of the elastomeric material, leaving half, or some part together with the lower metallic ring.